Discovery of antibodies which apparently interfere with embryonic nutrition and the important finding that the majority of amino acids utilized for protein synthesis by the embryo during organogenesis stem from serum proteins degraded by the visceral yolk sac (VYS) rather than from free amino acids have focused our attention on aspects of fetal nutrition and the changing roles of the VYS and chorioallantoic placentae of the rat during gestation. Our specific aims in this proposal are: 1) to determine the in vivo fractional rates of total protein synthesis, net protein synthesis (growth) and protein degradation during organogenic and histogenic periods of gestation and the effect of teratogens, i.e., anti-rat VYS serum (AS), trypan blue (TB), and leupeptin (LP), on these parameters in the rat; 2) to determine the relative contribution of serum proteins versus serum free amino acids utilized for fetal protein synthesis in vivo during organogenic and histogenic periods of gestation and the effect of AS, TB and LP on these parameters; and 3) to determine relative net incorporation of amino acids stemming from serum proteins and serum free amino acids into fetal rat proteins which were supplied to the fetus via the chorioallantoic placenta versus the amino acids supplied via the VYS placenta in vitro and the effect of AS, TB, and LP on these parameters. Our studies will provide evidence, during a wide range of gestation, of changes in the kinetics of fetal protein metabolism, of changes in the relative importance of exogenous amino acids versus serum proteins as sources of amino acids, of changes in the relative magnitude of amino acid nutrition support of the fetus by the two placentae, and how fetal malnutrition resulting from placental dysfunction may play a primary role in teratogenesis induced by AS, TB, and LP.